1. Field of the Invention
The present invention relates to a signal processing circuit of a solid state image pickup device in which the noise of the signal voltage output of the solid state image pickup device can be minimized and the drift of the signal output by the thermal influence in the device can be also suppressed.
2. Related Background Art
Hitherto, a solid state image pickup device is provided with: a signal voltage output circuit to output a charge signal as voltage information through a charge-voltage converter; and a correction voltage output circuit to generate a reference voltage into which no signal charge is input and which has a circuit arrangement similar to the signal voltage output circuit. This solid state image pickup device, in general, uses the driving system such that the noise of the in-phase component is eliminated by obtaining the difference between the output signal of the correction voltage output circuit and the output signal of the signal voltage output circuit. When the dynamic range of the light information is about 10 to 10.sup.2, a considerable large signal output can be also obtained by the conventional driving system, so that it can be used for the automatic focus control of the camera and for the signal process of the facsimile apparatus and the like.
This point will be further described in detail hereinbelow.
FIG. 1 is a graph showing time-dependent changes of a signal voltage output V.sub.os and a correction voltage output V.sub.dos after a power source was applied to the solid state image pickup device. According to the actual measured data, even after tens of minutes or more have elapsed, a fluctuation above a few millivolts per minute still remains. Therefore, this fluctuation fairly obstructs the measurement with a high degree of accuracy over a wide dynamic range.
FIG. 2 is a differential amplifier which has conventionally been used to eliminate the in-phase noises such as the time-dependent change and the like. In the diagram, reference character SP denotes a solid state image pickup device; R.sub.10 to R.sub.13 are operational resistors; and OP is an operational amplifier. These components constitute the differential amplifier between the signal voltage output V.sub.os and the correction voltage output V.sub.dos. The signal of V.sub.dos - V.sub.os is output to the output terminal of this differential amplifier. Therefore, the influence by the time-dependent change, for example, as shown in FIG. 1 can be eliminated.
However, when the dynamic range of the light information is above 10.sup.2 to 10.sup.3, the influences of the respective random noises of the signal voltage output circuit and correction voltage output circuit cause a problem. If the difference between the correction voltage output and the signal voltage output is simply obtained as in the conventional system, the noises will in turn increase, causing the S/N ratio to deteriorate.